1. (Read Forouzan Chapters 6 and 7)) IP Forwarding Procedure B
Example Network:
/28
R A
eth1
/26
R C
/28
R B
eth0
/27
/27
R D A
/27
IP Addresses Identify an
Interface not a machine
(Reference : “ IP Fundamentals” by Maufer Part II)

2. In Sending An IP Packet From A to B
STEP 1:
Since B [ ]00101
and A’s network /27 [ ]00000
do not match in masked off significant part (Prefix):
A must send IP packet to a default gateway which A must know (by set up) to be RB
A must send Address Resolution Protocol (ARP) request to find out RB MAC.

3. … … A then sends IP SA IP Destination Address IP Packet IPA IPB
IP packet data … …
MAC DA MAC SA
FCS
MAC Frame
RB ( MAC)
MACA
0X0800
Ether
Type
Frame Control
Sequence ( CRC )
STEP :
At RB IP frame is extracted and IP Destination Address is examined and compared for longest match
in a forwarding table.

4. For Example:
Known Prefixes Next – Hop Gateway Interface Metric / /
/ eth0 / / /
Later
The destination prefix that has the most leading bits in common with the IP DA is
/26 =
/26 = [ ]000000
Prefix

5. STEP 2 :
This means we need to forward the packet to whoever has as an IP address.
CAUTION: we want to forward at the MAC layer so need to use ARP to find MAC address
for DO NOT CHANGE IP DA!!!!!
New time to live => new IP
header check sum
IP SA IP DA IP
IPA
IPB
IP Packet Data
Different value
from step 1
MAC DA MAC SA
MAC
RC MAC
RB MAC
0x0800
FCS
ether
Type
Frame Control
Sequence (CRC)

6. match in a forwarding table.
STEP 3 :
At RC IP frame is extracted and IP destination address is examined and compared for longest
match in a forwarding table.
For Example:
Known Prefixes Next – Hop Gateway Interface Metric / /
/ eth1 / /
Later
Longest prefix match of same old IP DA is with /26
Difference from before - since RC is directly connected to destinations subnetwork, the subnetwork
layer destination MAC address MACB and IP destination address ( IPB ) are for the same machine.

7. IP SA IP DA
IP Packet
Data
IPA
IPB
MAC DA MAC SA
FCS RC
(MAC)
MAC B
0X0800
Ether
Type
Aside: Routing protocols ( RIP or OSPF ) build forwarding tables (or can do by hand )

8. What is the difference between routing and switching?
LAN interconnection devices operate a “ OSI ” (Open Systems Interconnection Reference Model)
Layers:
Layer 1 : Physical Layer
Layer 2: Data Link Layer
Layer 3: Network Layer
Application
Presentation
Application
Session
Transport
Transport
Internet
Network
Subnetwork
Data Link
Physical
Open Systems
Interconnection
Reference Model
Internet Protocol
Stack

9. Layer 1:
Repeaters ( Not too common now)
HUBS ( Very common )
HUB
To Backbone Four HUB rule - only 4 hub hops allowed in a
“ Collision Domain ”
Hubs are physical layer devices that just “Repeat” what they see, errored frames and all.
Endstation
TD+ 
TD- 
RD+  4 5
RD-  7 8
Hub Side
TD+
TD-
RD+ 4 5
RD- 7 8
Endstation
TD+ 
TD- 
RD+  4 5
RD-  7 8
Endstation
1  TD+
2  TD-
3  RD+ 4 5
6  RD- 7 8
10 Base-T pinout and Crossover cable
HUB attachment Functionality

10. Layer 2:
Bridges ( Not common now )
Switches ( What we call bridges now )
Layer - 2 switches ( Way cool marketing term)
Forwarding decisions based only on data link layer header, that is the MAC DA. Use a table made from
observing which addresses are seen on each port.
No more than 7 bridges in diameter.
Today’s ethernet environments use lots of ethernet switches, reducing the collision domain.
Collision domain is the set of ethernet segments that can directly hear each others frames.
Worst case is all inputs want to go out on the same output. Must have some buffering
and some sort of fairness algorithm inside

11. Layer 3: routers
( Historically Called a Gateway When Different Protocols Were Involved )
Routers forward packets based on the destination address at network layer ( Layer 3 ) 3 2
LAN 1
LAN 2 1
May see “Layer - 3 switching” term this just means Routing !

12. Early Routing
Initially called gateways ( Gateway From One Network To Another ) and later called routers.
Used to connect different physical networks into larger unified network.
Packets contain a destination address, router attempts to match with one of many possible entries
in a table of destination networks, sends out appropriate interface.
First generation router architecture:
Processor
Shared Bus
Interface
Interface
Interface
Processor runs routing protocols and maintains forwarding table of next HOP routers
Packets flow from interface up to processor then back down to correct interface
Performance limited by speed of the bus and processing capability of CPU.
Second generation router:
Distribute the forwarding computations out to the media interface adapters.
Input adapter performs forwarding computation and directs the packet over bus to appropriate output
adapter.
Performance still limited by speed of the bus.

13. Third Generation Router:
Replace bus with a switch. All ports connected by nonblocking switch fabric.
Processor
Forwarding
Cache
Forwarding
Cache
Adapter
Adapter
Forwarding
Cache
Forwarding
Cache
Adapter
Adapter
Reference: “IP switching protocols and architectures” by Metz Chapter 1